H. B. Newton — A Jurassic Shell from Somali-land. 295 



for determination. The specimens, belonging to one species, are 

 of various sizes, quite black in colour, and covered in places with 

 a lightish-brown matrix. 



Externally they resemble Byssoarca, having very anterior, distant, 

 and incurved umbones, with a wide ligamental area between, through 

 the centre of which runs a long and straight hinge-line. An exami- 

 nation of the interna] characters of the hinge-plate, obtained through 

 sectioning one of the specimens (the valves of all the examples being 

 united), reveals the presence in the left valve of seven or eight 

 small, parallel, and oblique teeth at the anteinor end of the hinge ; 

 and succeeding these are two long rib-like teeth, arranged parallel 

 with the hinge-line and extending to its posterior termination. 



The exposure of this dentition has enabled me to refer the shells 

 to the genus Parallelodon of Meek and Worthen, a name substituted 

 by those authors for the better known Macrodon of Lycett, which 

 had, unfortunately, been previously used by M tiller for a fish. 

 When Lycett first diagnosed this genus it was only known from 

 the Inferior Oolite of the Cheltenham neighbourhood, his type being 

 called Macrodon rugosus. Since this, however, it has been discovered 

 in much older rocks, and we now find it recorded as ranging; from 

 Devonian times to the present day, recent examples having been 

 described from the seas of the Antilles 1 and Japan. 2 As the 

 Secondary shells of Africa frequently present a strong Indian facies, 

 a comparison had to be instituted before the Somali species could be 

 definitely named or its horizon determined. 



Among the Northern Himalayan fossils in the British Museum, 

 two shells were detected corresponding in every detail with those 

 from East Africa, and clearly referable to Stoliczka's Macrodon Eger- 

 tonianus. This species was first figured by Everest in 1833, as an 

 Area ; subsequently Blanford mistook it for the Cutch form of 

 Cucullcea virgata, J. de C. Sowerby ; and still later it was recognized 

 as entirely new by Stoliczka, when it received the name of Macrodon 

 Egertonianus. 



The Niti rocks and Spiti shales of the Northern Himalayas, which 

 have both yielded this species, besides such well-known forms as 

 Trigonia costata, Rhynchonella varians, Camptonectes lens, Stephauo- 

 ceras Braihenridgei, Parkinsonia Parkinsoni, etc., are regarded as 

 of Lower Oolite or " Dogger " age ; we may therefore safely assign 

 the African representative of this species to the same position in the 

 Jurassic series. 



Genus Parallelodon, Meek and Worthen. 



Proceedings Chicago Academy, 1866, vol. i, p. 17. 



Macrodon, Lycett [non Miiller, 1842], Murchison's "Geology of 

 Cheltenham," 1845, pi. v, fig. 5, pp. 98, 99. 

 Type — Macrodon rugosus, Lycett. 



1 Macrodon asperula, Dall, Bull. Mus. Comp. Zool. Harvard, 1881, p. 120 ; ibid., 

 1885, pi. viii, fig. 4, p. 244. 



3 Macrodon Dalli, E. A. Smith, "Challenger" Eeport [Laniellibranchiata], 1885, 

 pi. xvii, fig. 10, p. 269. 



